This AU needs spriters!
Mutantale is an Alternate Universe where almost everyone has been mutated in one way or another.
Long after the Monsters were sealed into Mt. Ebott, another monster awakened. He was almost exclusively green, save for his hands, feet, face and jaws. He could mutate anyone except spirits and skeletons. Although he could still prevent ghosts from turning invisible/intangible and melt skeletons with his acid. This beast was known as "Mutomination".
He had mutated many into his slaves. As Mutomination reached the capital, he had amassed an army. The people of the Capital did everything they could to fend off the attackers. As Mutomination walked towards King Asgore Dreemurr, Asgore picked up a rock in defense. Mutomination put his hands up to shield himself. Asgore realized this type of rock had an adverse effect on the creature so he walked towards Mutomination with the stone in front of him. Mutomination ran away, and Asgore and some others chased after the beast. The best eventually reached an empty cave in Waterfall and quickly hid in it. Asgore and the others then put the lodestones at the mouth of the cave.
Much, much more to be added.